1. Field of the Invention
This invention relates to method and apparatus for dispensing a fluidized stream of particulate material and more particularly a blended mixture of particulate materials, which mixture is formed during transport in a line extending to the receiving vessel. The invention more specifically relates to method and apparatus for controlling mixing and dispensing of desulfurizing agents for the desulfurization of molten ferrous metals.
2. Description of the Prior Art
This invention is an improvement of the invention disclosed and claimed in U.S. Pat. No. 3,998,625, which is owned by the assignee of this invention and which is incorporated by reference herein.
U.S. Pat. No. 3,998,625 discloses a desulfurization process in which a particulate non-oxidizing material such as lime and particulate magnesium-containing material are separately fed from their respective storage means to form a fluidized mixture in a non-oxidizing carrier gas and this mixture is injected into a molten ferrous metal. The magnesium component of the injected mixture serves as a potent desulfurization agent in the ferrous metal. A principal advantage of the process taught in U.S. Pat. No. 3,998,625 is that the injection rate of magnesium-containing material may be varied during the injection period to take into account process variables such as the fact that the efficiency of magnesium desulfurization decreases as the sulfur content of the bath decreases.
U.S. Pat. No. 3,807,602 describes a process for dispensing a fluidizable solid from the lower portion of a pressure vessel equipped with a weighing device, a closable supply, a discharge, and a source of three gas flows; a first of which is applied within the vessel at a level above the solid to be fluidized; a second of which is applied within the vessel in a lower portion thereof; and a third of which is applied within the discharge orifice of the vessel. A control device is associated with the weighing device which compares the actual value given by the weighing device with a desired value and in accordance with the result regulates the pressure of the above described first gas flow.
A major problem which has confronted the art in this field has been the inability to keep the particulate materials flowing through the transport lines in positive even flow when variations in gas flow pressures are encountered. It is difficult to maintain a constant even pressure in the fluidized stream of gas flow when the source of gas flow is being used to supply a gas flow to more than one dispensing unit or hopper and/or to more than one type of unit or line. The problem of maintaining desired gas flow pressures in a single dispensing unit is very complex. Hence, it will be appreciated that the problem of maintaining desired gas flow pressures in a plurality of gas flow lines amongst multiple dispensing hopper vessels feeding into a common transport line may be compounded at least exponentially. It should be recognized that in a dispensing unit there will occur, unless otherwise compensated for, a gradually decreasing flow resulting from the automatic reduction of the top space pressure in the hopper resulting from the reduction in the material volume and weight and from the increase in gas volume. In dispensing units, such as disclosed in the above referenced U.S. Pat. Nos. 3,998,625 and 3,807,602, which rely upon a separate gas flow to the top of a dispensing unit or hopper above the level of the material in the hopper to maintain positive flow of the material and upon regulation of the pressure of such separate gas flow, the pressure regulators are generally in constant operation in an attempt to adjust for the change in gas flow pressure caused by the material volume change. A concomitant problem with operator attendant controlled processes is one of time, i.e., the desulfurizing time cycle is very often completed before the pressures of the fluidized streams can be brought into proper balance, even by a highly skilled operator.
With this background, the present invention was conceived to overcome the undesirable effects of variations in pressures with the subsequent disruption of the smooth flow of particulate material, e.g., desulfurizing agents, from dispensing units through a transport line, and more particularly to overcome the undesirable effects of variations in gas flow pressures amongst a plurality of desulfurizing agent dispensers feeding into a common transport line for in-line mixing and subsequent feeding to a vessel containing molten ferrous metal.